Eureka3 User Manual
Eureka3
Page 7
2 Principles of Leak Noise Correlation
Leak noise correlation is used to detect leaks in pipelines where there is a positive internal pressure,
such that a leak causes a loss of fluid to the outside. The movement of fluid through the leak causes
rapid pressure changes around the site of the leak such that there is a randomly varying noise source
located at the leak. This noise travels in both directions, at the same velocity, from the leak.
(Noise is not a constant frequency like a musical note, but is a random and continually changing
mixture of different frequencies. It is this characteristic which enables the correlator to operate.)
2.1
Methods of location
Previous methods of leak location relied on detecting a noise, then tracing it at ground level to find the
point of maximum noise, which was assumed to be directly above the position of the leak. The
equipment used included listening sticks and stethoscopes, but these were difficult to use if the noise
was faint or too loud to be accurately located. Background noise was another problem; electronically-
filtered amplifiers were introduced in an attempt to overcome such problems.
A correlator operates by comparing the noise detected at two different points in the pipeline. Noise
travels from the leak in both directions along the pipeline at a constant velocity (which depends on
various factors), so that if the leak is equidistant between two sensors then these sensors will detect
the noise at the same time. Conversely, if the leak is not equidistant then the sensors will detect the
same noise at different times, and the difference in time (the time delay) is measured by the correlator.
The following diagram illustrates this:
The sensors are located on the valves A and B (convenient access points for underground pipes), and
as shown the leak site is closer to A.
By the time a particular noise from the leak has reached A, the same noise heading towards B has
only travelled as far as X. The distance from X to B causes a delay t before the noise arrives at B, thus
the correlator detects the delay (t) between the arrival of the noise at A and its arrival at B. If the
velocity of sound is V and the distance between the sensors is D, then as the distance from X to B = V
* t, (velocity x time), then D = (2 * L) + (V * t). This equation may be rearranged to give L, the distance
from the nearer sensor to the leak site:
2
)
*
(
t
V
D
L
−
=
As the sound velocity can be calculated from knowledge of the pipe diameter and material, while the
distance between the sensors can be determined by careful measurement, the correlator can
calculate and display the location of the leak as a direct distance from the nearer sensor. Alternatively,
the correlator may be used to measure the actual velocity of sound in the pipework being examined,
so ensuring the highest possible accuracy in locating the leak.
2.2
Sensor Position
Correlator operation depends on having the leak located between the two sensors. There are two
situations where this is not the case and one sensor will have to be moved before an accurate
correlation can be performed.